Download force

A FORCE is a push or a pull on an object.
All forces have strength and direction.
The strength of a force is measured in Newtons (N).
A force results when two or more objects INTERACT with
each other. When the objects stop interacting, there is no
more force.
• Some forces result when two objects physically touch each
other. These forces are called CONTACT FORCES.
• Some forces result even when two objects do not physically
touch each other. These forces are called NON-CONTACT
FORCES.
•
•
•
•
Forces & Motion
1
© Stephanie Elkowitz
•
•
•
•
•
•
•
•
•
Applied
Friction
Drag (air resistance)
Elastic (Spring)
Tension
Normal
Buoyancy
Lift
Thrust
•
•
•
•
•
Magnetism
Gravity
Electromagnetic
Weak Nuclear
Strong Nuclear
Forces & Motion
CONTACT
FORCES
(have to touch)
NON-CONTACT
FORCES
(do not touch)
2
• An applied force is a force applied by a person or object
onto another object.
• An applied force can change the motion of an object. It can
cause an object to move in the same direction as the force.
It can also slow or stop a moving object.
Forces & Motion
3
© Stephanie Elkowitz
• Friction is a force that opposes motion. It works in the
opposite direction of a moving object.
• Friction is a force you must overcome to move a stationary
object.
• Friction is a force that causes moving objects to slow down.
Forces & Motion
4
© Stephanie Elkowitz
• The force of friction exerted by a surface depends on the
smoothness of the surface.
• A smooth surface exerts less friction than a rough surface.
• The surface of objects can be coated with liquid to reduce
friction. Liquid makes the surface smoother. This is why oil
is important to a car engine. The oil decreases friction
between the rubbing parts in the engine.
Forces & Motion
5
© Stephanie Elkowitz
• Only solids exert friction.
• Gases and liquids resist
motion. This resistance is
called drag.
• Air resistance is a type of
drag. It is like “air friction.”
• The direction of air
resistance opposes the
direction of motion.
• Air resistance slows falling
objects. It also slows an
object moving through air,
like planes and cars.
Forces & Motion
6
© Stephanie Elkowitz
• Air resistance does NOT depend on the mass of an object.
• Air resistance depends on the speed, shape and orientation
of an object moving through air.
• A fast moving object experiences more air resistance than a
slow moving object.
• An object shaped and orientated so it has more surface
area in contact with air experiences more air resistance.
• Planes are streamlined to reduce air resistance. This allows
them to fly faster through the air.
• Parachutes are large so to “capture” more air resistance.
Parachutes slow the downward movement of an object
through air.
Forces & Motion
7
© Stephanie Elkowitz
• Gravity, or gravitational
force, is a force of
attraction between two
objects.
• All objects with mass exert
a gravitational force.
• Larger objects exert a
greater gravitational force.
• We only notice the
gravitational force of very
large objects, such as
stars and planets.
Forces & Motion
8
© Stephanie Elkowitz
• Gravity is the force that attracts objects to Earth. It pulls
objects towards the center of Earth.
• Earth’s gravity also keeps the moon in orbit around Earth.
Forces & Motion
9
© Stephanie Elkowitz
• The force of gravity decreases when the distance between
objects increases. This explains why the moon does not fall to
Earth’s surface, but objects in Earth’s lower atmosphere do. The
moon is 238,900 miles away from Earth! At this distance, the
strength of Earth’s gravity is strong enough to keep the moon in
orbit but not so strong that the moon crashes to Earth’s surface.
Why doesn’t
gravity cause
the moon to
crash into
Earth?
Forces & Motion
10
© Stephanie Elkowitz
• The sun’s gravity keeps Earth and other planets orbiting
around sun. The Earth is at the perfect distance from the
sun (based on its mass and other factors). The sun’s gravity
keeps Earth in orbit but does not pull Earth so much that
the planet crashes into the sun.
Forces & Motion
11
© Stephanie Elkowitz
• In addition to his 3 laws of motion, Isaac Newton is
credited with the discovery that gravitation is universal.
He stated that ALL objects attract each other with a
force of gravitational attraction. Gravity is universal.
• This force of gravitational attraction is directly
dependent upon the masses of both objects and
inversely proportional to the square of the distance
that separates their centers OR (translation) :
Forces & Motion
12
To put it more simply….
Forces & Motion
13
Last year, you were taught
about the Four Fundamental
Forces in Nature.
In each box, read the relevant
examples of where you find
these!
Forces & Motion
14
• Weight is the result of gravity pulling on an object.
• Weight is NOT the same as mass.
• Mass is the amount of matter in an object. Mass is
measured in kilograms (kg).
• Weight is a measure of gravity’s effect on mass. Weight is
measured in Newtons (N).
Forces & Motion
15
© Stephanie Elkowitz
• The force of gravity is much less on the moon because the
moon is much smaller than Earth. Because gravity is less, a
person’s weight is less. A person can jump higher using the
same amount of force as he would use on Earth since he
has to overcome a lesser force of gravity (weight).
Why can an
astronaut
jump so
high on the
moon?
Forces & Motion
16
© Stephanie Elkowitz
THINK ABOUT IT...
The force of gravity is greater on Jupiter because Jupiter is
larger than Earth.
Would a person’s weight on Jupiter be greater, less than or the
same as his weight on Earth?
Would a person’s mass on Jupiter be greater, less than or the
same as his mass on Earth?
Forces & Motion
17
© Stephanie Elkowitz
THINK ABOUT IT...
The force of gravity is greater on Jupiter because Jupiter is
larger than Earth.
Would a person’s weight on Jupiter be greater, less than or the
same as his weight on Earth?
IT WOULD BE GREATER.
Would a person’s mass on Jupiter be greater, less than or the
same as his mass on Earth?
IT WOULD BE THE SAME.
Forces & Motion
18
© Stephanie Elkowitz
You can calculate weight using the equation:
Weight (Fg) = Mass (m) × Gravity (g)
Gravity on Earth is 9.8 m/s2
Example:
Mass = 10 kg
Gravity = 9.8 m/s2
Weight = Mass × Gravity
Weight = 10kg × 9.8 m/s2
Weight = 98 N
Forces & Motion
19
© Stephanie Elkowitz
TRY IT:
What is the weight of a 50 kg person on Earth? On the moon?
Formula: Fg = m × g
Gravity on Earth = 9.8 m/s2
Gravity on Moon = 1.6 m/s2
Forces & Motion
20
© Stephanie Elkowitz
TRY IT:
What is the weight of a 50 kg person on Earth? On the moon?
Earth:
Fg = m × g
Fg = 50 kg × 9.8 m/s2
Fg = 490 N
Moon:
Fg = m × g
Fg = 50 kg × 1.6 m/s2
Fg = 80 N
Formula: Fg = m × g
Gravity on Earth = 9.8 m/s2
Gravity on Moon = 1.6 m/s2
Forces & Motion
21
© Stephanie Elkowitz
• A magnetic force is an invisible force created by magnets. It
is a force that pushes or pulls magnetic objects towards or
away from the magnet.
• The area of a magnetic force around a magnet is called the
magnetic field.
Forces & Motion
22
© Stephanie Elkowitz
• An electric force is an invisible force created by electrically
charged objects. Objects with an electric charge are
attracted to or repelled by other objects with an electric
charge. This attraction and repulsion is called electric force.
• The area of electric force around an electrically charged
object is called an electric field.
Forces & Motion
23
© Stephanie Elkowitz
• Tension is created when two objects pull on a rope, string,
wire or cable in opposite directions.
• Tension is the force created in the wire when objects pull on
the wire. Tension pulls on the objects equally towards the
center of the wire.
• If a person pulls on a wire anchored to a wall, tension in the
wire pulls back on the person towards the wall.
Forces & Motion
24
© Stephanie Elkowitz
• Spring force is a force created by a stretched or compressed
spring.
• When a spring is compressed, it wants to push outwards to
its neutral/resting position.
• When a spring is stretched, it wants to pull inward to its
neutral/resting position.
Forces & Motion
25
© Stephanie Elkowitz
• A fluid pushes upward on an
immersed object. This force is
called buoyancy or buoyant force.
• If buoyancy is equal to or greater
than the weight of an immersed
object, the object will float.
• If buoyancy is less than the weight
of the object, the object will sink.
• Buoyant force is directly related to
the density of the fluid and how
much fluid is displaced, or moved,
by an immersed object.
Forces & Motion
26
© Stephanie Elkowitz
• The normal force is a force exerted by
a surface on an object resting on that
surface.
• On a level surface, the normal force
is equal and opposite to the weight of
the object.
• The normal force explains why a book
resting on a table does not move. The
force of gravity (weight) pulls the
book down to the surface. A force
acts in an equal and opposite
direction so that the book does not
move. This force is the normal force.
Forces & Motion
27
© Stephanie Elkowitz
• When the forces on an object are equal and balanced, the
object’s motion does not change. If the object is at rest, it
will stay at rest. If the object is moving, it will continue
moving in the same direction with the same speed.
Forces & Motion
28
© Stephanie Elkowitz
• When the forces on an object are NOT equal and balanced,
the object’s motion will change. The objects speed, position
and/or direction will change.
• The change in an object’s motion depends on the net force
acting on the object and the mass of the object.
Forces & Motion
29
© Stephanie Elkowitz
• The combined result of all forces acting on an object is
called the net force.
• If two forces are acting in opposite directions, the net force
is the difference between the two forces.
Net Force = 10 N – 5 N
Net Force = 5 N to the RIGHT
Forces & Motion
30
© Stephanie Elkowitz
• The strength of force is directly related to an object’s mass
and acceleration.
• You can calculate the force of an object using the equation:
Force (F) = Mass (m) × Acceleration (a)
• This equation tells us the force of an object depends on
how massive the object is and how much the object is
accelerating.
• Objects with a greater mass have greater force.
• Objects with a greater acceleration have greater force.
Forces & Motion
31
© Stephanie Elkowitz
A truck has a mass of 2,000 kg. It is accelerating 20 m/s2.
What is the truck’s force?
A car has a mass of 1,500 kg. It is accelerating at 20 m/s2.
What is the car’s force?
Forces & Motion
32
© Stephanie Elkowitz
A truck has a mass of 2,000 kg. It is accelerating 20 m/s2.
What is the truck’s force?
F=m×a
F = 2,000 kg × 20 m/s2
F = 40,000 N
A car has a mass of 1,500 kg. It is accelerating at 20 m/s2.
What is the car’s force?
F=m×a
F = 1,500 kg × 20 m/s2
F = 30,000 N
Forces & Motion
33
© Stephanie Elkowitz
• Motion is the movement of an object.
• An object moves when unbalanced forces act on the object.
• Pushing or pulling an object will change an object’s position
and/or direction.
Forces & Motion
34
• The motion of an object is described with respect to some
other object or position.
• The motion of an object is described by its position,
direction of motion and speed.
– Position: on top of, next to, over, under
– Direction: up/down, left/right, north/south
– Speed: miles per hour (mph), meters per second (m/s)
Forces & Motion
35
• Velocity describes the speed and direction of an object’s
motion (it is a vector quantity…a value and direction)
• Speed is the distance traveled in a certain amount of time
(it is a scalar quantity…only a value (no direction)
• Direction is the way or path an object moves
• You can calculate velocity using the equation:
velocity (v) = distance (d) ÷ time (t)
• Velocity is measured in meters/second (m/s)
EXAMPLE:
A car travels east 100 meters in 2 seconds.
Velocity = distance ÷ time
Velocity = 100 meters ÷ 2 seconds
Velocity = 50 m/s east
Forces & Motion
36
• Acceleration describes the change in an object’s velocity
• Objects that speed up have a positive acceleration
• Objects that slow down have a negative acceleration
(this is also called deceleration in BAD science)
• You can calculate acceleration using the equation:
acceleration (a) = change in velocity (v) ÷ time (t)
• Acceleration is measured in meters per second2 (m/s2)
• Acceleration is a vector quantity
EXAMPLE:
A plane’s velocity changes from 0 to 100 m/s in 5 seconds while traveling west.
Acceleration = change in velocity ÷ time
Acceleration = (100 m/s – 0 m/s) ÷ 5 seconds
Acceleration = 20 m/s2 west
Forces & Motion
37
•
•
•
•
Graphs can be used to describe the motion of an object
A distance vs. time graph shows velocity
A velocity vs. time graph shows acceleration
What do the following graphs show?
Forces & Motion
38
Zero velocity
because there’s no
change in distance
over time
Constant velocity
because distance
directly increases
over time
Increasing velocity or
acceleration because
distance exponentially
increases over time
Zero acceleration
because velocity
does not change
over time
Positive acceleration
because velocity
increases over time
Negative acceleration
(deceleration)
because velocity
decreases over time
Forces & Motion
39
• Isaac Newton was a
scientist and
mathematician who lived
1643 – 1727.
• He developed three laws of
motion to describe how
forces interact with objects
and cause motion.
• Newton also made
important findings about
gravity and how to calculate
the gravitational force
between two objects.
Forces & Motion
40
© Stephanie Elkowitz
An object at rest will
remain at rest unless
acted on by unbalanced
forces. An object in
motion continues in
motion with the same
speed and direction
unless acted on by
unbalanced forces.
Forces & Motion
41
What does this mean?
This means that objects
want to keep on doing
what they are doing.
Objects resist changes to
their state of motion. If
there are no unbalanced
forces, an object will
maintain its state of
motion.
© Stephanie Elkowitz
• Newton’s 1st law is also called the “Law of Inertia.”
• Inertia is the tendency to resist change in motion.
• Inertia explains why it takes time for a car to come to a
stop. A car moving forward wants to continue its motion.
When the driver pushes on the breaks, the car and the
passengers inside the car want to continue moving forward.
Forces & Motion
42
© Stephanie Elkowitz
• Momentum is the quantity of motion an object has.
• The momentum of an object depends on the object’s mass
and velocity.
• A heavy and fast moving object has more momentum than a
lightweight and slow moving object.
• More force is needed to change the motion of a heavy and
fast moving object than a lightweight and slow moving object
because it has more momentum.
Forces & Motion
43
© Stephanie Elkowitz
Two football players are running
down the field with the same
speed. One player has a mass of 70
kg. The other has a mass of 100 kg.
Which player has more
momentum?
Which player is harder to tackle?
Forces & Motion
44
© Stephanie Elkowitz
Two football players are running
down the field with the same
speed. One player has a mass of 70
kg. The other has a mass of 100 kg.
Which player has more
momentum?
The 100 kg player.
Which player is harder to tackle?
The 100 kg player because he has
more momentum.
Forces & Motion
45
© Stephanie Elkowitz
An object accelerates
when a force acts on an
object with mass. The
greater the mass of the
object being
accelerated, the more
force needed to
accelerate the object.
What does this mean?
This means that more
force is needed to move
heavier objects. This law
also explains what
happens when you apply
an equal force to a heavy
and a lightweight object –
the lightweight object
moves (accelerates) more.
This law establishes the
equation F = ma.
Forces & Motion
46
© Stephanie Elkowitz
• Example: If a boy applies the same force to each wagon, the
wagon that has twice as much mass will accelerate half as
fast. It would take twice the amount of force to accelerate
the wagon with 20 kg the same as the wagon with 10 kg.
Forces & Motion
47
© Stephanie Elkowitz
For every action
there is an equal
and opposite
reaction.
Forces & Motion
What does this mean?
This means there is a
force equal in size but
opposite in direction for
every force. In other
words, when one object
pushes on a second
object, the second object
pushes back on the first
object in the opposite
direction equally hard.
48
© Stephanie Elkowitz
• Example: When a rocket
blasts off, the force of its
powerful engines pushes
down on Earth’s surface.
This is the action. The
reaction is that Earth’s
surface pushes the rocket
upward with an equally
strong force. This causes
the rocket to move upward
into space.
Forces & Motion
49
© Stephanie Elkowitz
• A collision is an interaction between two objects that
physically come into contact with each other.
• A collision does not necessarily involve an accident – it is
any event where two objects bump into each other.
• Newton’s 3rd Law describes what happens during a collision.
The force exerted by one object is equal and opposite to the
force exerted by the second object.
• Example: If a pool stick collides
with a pool ball, the force
exerted by the stick onto the
ball is equal and opposite to
the force exerted by the ball
onto the stick.
Forces & Motion
50
© Stephanie Elkowitz
• Momentum is conserved during
a collision. This means the total
momentum of the two objects
before the collision equals the
total momentum of the two
objects after the collision.
• If the mass of each object stays
the same, the velocity of the
objects must change. This
explains why the speed and/or
direction of movement changes
for one or both objects during a
collision.
Forces & Motion
51
Remember...
The momentum of an object
depends on mass and velocity.
© Stephanie Elkowitz
Car Accidents (Inertia)
• During a car accident, the vehicle
(and passengers in the vehicle)
have inertia. When a car comes
to an abrupt stop, the vehicle and
passengers in the vehicle want to
continue moving forward. The
vehicle will crumple against the
object(s) it crashes into to, forcing
it to come to a stop. However,
passengers will continue to move
forward. Seatbelts help keep
passengers from being ejected
from the vehicle. Seatbelts apply
a force against passengers so
they stay within the vehicle.
Forces & Motion
52
© Stephanie Elkowitz
Airplanes (Unbalanced Forces)
• Airplanes are able to fly because
of the shape of their wings.
When a plane propels forward,
the wings move through the air.
Air that moves under the wing
creates an upward force called
lift. The faster the plane moves,
the greater the upward force
(lift). When lift is greater than
the force of gravity acting on the
plane (weight), the plane
elevates in the sky. Airplanes
adjust their speed and the
shape of the wing to rise, stay
steady or lower in the sky.
Forces & Motion
Gravity
(weight)
Lift
53
© Stephanie Elkowitz
Space Rockets (Newton’s 3rd Law)
• Rockets provide an excellent example
of Newton’s 3rd Law of Motion. The
engine of a rocket creates a strong,
downward force toward the surface of
Earth. This action is counteracted by a
reaction. The ground exerts an equally
strong but opposite force on the
rocket. This reaction causes the
rockets to move upward, through the
atmosphere and into space.
Forces & Motion
54
© Stephanie Elkowitz
Life Jackets (Buoyancy)
• Life jackets or vests help keep a
person afloat in water. A life
jacket is filled with trapped air.
When the jacket is submerged in
water, the jacket displaces some
water. The trapped air in the life
jacket weighs much less than the
water it displaces. So, water
pushes up harder than the life
jacket pushes down. This creates
buoyancy. When worn by a
person, the life jacket essentially
decreases the weight of a person.
It provides buoyancy (buoyant
force) to keep the person afloat.
Forces & Motion
55
© Stephanie Elkowitz
G-Force (Gravity)
• Some amusement rides, including
rollercoasters, accelerate
extremely fast, giving riders the
feeling of heaviness. This feeling
created by high acceleration is due
to a force called g-force. One
g-force is equal to the force of
gravity at Earth’s surface. An
amusement ride that produces a
force twice that of gravity is said to
produce 2 g-forces. Some rides,
including the Gravitron, produce
more than 3 g-forces. Flights into
space are known to produce more
than 7 g-forces!
Forces & Motion
56
© Stephanie Elkowitz
• Images obtained from commons.wikimedia.org and the
Public Domain
• Clipart by:
– Stephanie Elkowitz
– www.mycutegraphics.com
– www.mysweetclipart.com
Atoms & Reactions
57
© Stephanie Elkowitz